In the process of converting bauxite ore into alumina, electricity is passed through a bath by means of large carbon blocks which act as anodes. These blocks, normally two to four in number are affixed to a metal holder located above the bath so that the blocks depend into the bath to provide electrical conduction. In use, the blocks erode and require periodic replacement. In a typical alumina production plant, several hundred carbon blocks are consumed each day. Because of this, the blocks are usually manufactured on-site where they are consumed. In fact, entire departments are devoted entirely to the pressing and baking of carbon blocks.
It is desirable for the carbon blocks to have as long a service life as possible. However, the primary factor which adversely affects the service life is erosion, usually due to improper baking. In making a block, which usually weighs between about 350 and 550 lbs., carbon particles are pressed in a large ram press and are bonded together by a tar-like bonding agent. The block forms are then placed into a baking oven where the volatile binding agent is driven off a the carbon block is baked. Usually, the baking process occurs over about a 14- to 28-day cycle.
The typical carbon block baking oven comprises a large oblong rectangular pit, almost the length of a football field, with a poured concrete bed and walls contained within a corresponding large shed, or building. The pit is subdivided by a series of vertical crosswise refractory headwalls and lengthwise flue walls to form a battery of baking chambers in columns and rows in which the blocks are stacked and covered by loose granular material. Heat is then applied by conduction and radiation through the flue walls. Each flue wall is fabricated of laid up refractory bricks on either side with a flue space between communicating end-to-end with adjacent walls through apertures in the headwalls. A cover on the flue wall includes a series of lengthwise-separated ports, sealed by removable caps, for receiving fuel oil or gas burners which direct flame downwardly into the flue space. Cracks between the refractory bricks forming the flue walls permit volatiles given off from the carbon block bonding agents to pass into the flue space where they are ignited by the hot gases from the burners and provide additional heat for the baking process. The combined hot gases from the burners flow through row to row of lengthwise walls and the apertures in the headwalls and exit at the opposite end of the pit. The burners are periodically advanced lengthwise of the pit as the baking cycle progresses in a manner well known in the art.
The construction of a carbon baking oven as aforedescribed is a labor-intensive undertaking. Current practice in some installations involves the construction of each wall, including the flue walls, in situ in the pit. Depending on the size of the flue wall, each may comprise between 1,000 and 2,000 refractory bricks, and 16 to 30 man-hours to build. Considering that the construction must be undertaken in cramped space, and under dusty and dirty conditions, such a mode of construction is not as efficient as desired.
In other installations, the flue walls are constructed at one end of the building housing the pit, or near where they will be used, and transported to the pit where they are lowered into place. This necessitates the use of extra heavy-duty cranes and lifting equipment to transport the wall. In addition, the all-brick construction must be supported under the very bottom to ensure against fracture when lifted into place. The wall is therefore built on a steel member which is usually left in place after the wall is set. Sometimes these members can be retrieved and reused after a wall is removed for replacement.
The service life of a typical oven installation constructed as described above ranges from two to fifteen years. Generally, failure occurs most frequently in the flue wall. Failure is not usually due to refractory disintegration, but rather because the walls lose integrity and start to bow sideways. This bowing closes the space between the flue walls, and where the gap is closed too much, there is insufficient room for placing the carbon blocks for baking. Bowing of one wall will also cause an adjacent wall to bow very similar to a domino effect.
The major factor contributing to flue wall life depends on how quickly the pit is turned around. Turnaround time is the time from when the pit is emptied to when it is again refilled with carbon blocks and refired. Usually, the shorter the turnaround time, the shorter will be the service life of the flue wall as there is no time to repair any cracks or do any maintenance on the wall. This is because each cycle involves heating and cooling the refractories to remove the blocks.
Depending upon the capacity of the alumina production facility, and the through-put of the baths which consume the carbon blocks, some alumina production facilities have baking ovens which have short service lives. For instance, when alumina production is at maximum, anodes are consumed frequently, thereby requiring more frequent replacement. This, in turn, requires increased production of carbon blocks. If baking capacity is limited, blocks may be baked for shorter periods or at higher temperatures in order to keep up with the requirement for finished blocks. Unfortunately, improperly cured blocks erode quicker, thereby increasing the demand for additional replacement blocks, the result being a process which is akin to a faster and faster running treadmill. Heretofore, alternatives have been cost prohibitive because they have involved either the construction of additional pits or the purchase of carbon blocks from outside vendors. When it is considered that an average alumina production plant can consume between 250 and 400 carbon blocks per day, and the present market value for carbon blocks is between about $350 and $550 each, it should be apparent that there is a need for another solution to this vexing problem.
Owing to labor-intensive requirements of refractory block installations, costs, service life and lost production due to shutdowns, prefabricated refractory modules have evolved to replace the brick flue walls. The flue walls are assembled in sections from interrelated parts pre-cast in monolithic refractory concrete. However, each part in a section is unique in order to form the various configurations of baffles, ports and crossties. Thus, many casting molds may be required to form the parts necessary for a complete flue section. In addition, the shapes of these parts often manifest relatively complex molds.